The present invention relates to a call alarming apparatus for a paging system (pager) and, more particularly, to a drive apparatus for a vibration alarming motor (to be referred to as a VIB motor hereinafter).
A VIB motor is generally used as follows. That is, when a pager is individually called with a radio signal, the VIB motor is driven by a power supply battery to vibrate the pager itself or part of the pager, thereby alarming a call to the user of-the pager.
The arrangement of a VIB motor drive apparatus for a conventional pager is shown in FIG. 2. A controller 21 processes reception data from a receiver 20 when the receiver receives a self call number, and the controller 21 outputs an operation signal to a VIB drive circuit 22. The VIB drive circuit 22 comprises a current amplifier 24 for amplifying the current of the operation signal from the controller 21 and a switching transistor (to be referred to as a Tr hereinafter) 28 for ON/OFF-controlling a VIB motor 25. Note that the VIB drive circuit 22 may be constituted by an integrated circuit. The VIB motor 25 is started by the voltage of a primary battery 23 under control of the VIB drive circuit 22. A booster 27 boosts the voltage of the primary battery 23, and applies the boosted power supply voltage to the receiver 20 and the controller 21.
A detailed operation of the VIB motor drive apparatus will be described below. When the controller 21 confirms an individual call, a signal of high ("H") level is input from an output port VBQ of the controller 21 to an input port VBI of the VIB drive circuit 22. The current amplifier 24 is operated as follows. That is, the current amplifier 24 amplifies the signal of high ("H") level of the input port VBI and performs current amplification using a current from the primary battery 23. The amplified current flows into the base of the Tr 28 to turn on the Tr 28, and the collector of the Tr 28 is set at low ("L") level, thereby starting the VIB motor 25.
As another prior art, as described in Japanese Patent Laid-Open No. 63-268323, the following method is also known. That is, detection of a large current generated during calling is performed, and only when the large current is detected, the VIB motor is driven.
In the above conventional VIB motor drive apparatus shown in FIG. 2, since the primary battery 23 commonly supplies power to the current amplifier 24 and the VIB motor 25, the following problem is posed.
That is, when an internal impedance r.sub.0 of the primary battery 23 is high, the VIB motor may not be driven. In recent years, although a coin-type air zinc battery is frequently used, since this battery has an impedance r.sub.0 higher than that of an alkaline battery, a manganese battery, or an Ni-Cd battery which has been conventionally used, the coin-type air zinc battery may not be able to start the VIB motor.
This problem will be described below using V.sub.CE (collector-emitter voltage)-I.sub.C (collector current) characteristics of the Tr 28 with reference to FIG. 3. Reference numeral 33 denotes a V.sub.CE -I.sub.C characteristic curve obtained by using a base current I.sub.B of the Tr 28 as a parameter. The base current I.sub.B of the Tr 28 increases or decreases depending on a power supply voltage Vcc of the current amplifier 24. The base current I.sub.B of the Tr 28 increases when the power supply voltage Vcc is higher, and the base current I.sub.B of the Tr 28 decreases when the power supply voltage Vcc is lower. Reference numeral 31 denotes the DC load line of the Tr 28, and the DC load line is a load line obtained by connecting an open-circuit voltage V.sub.0 (indicated by reference numeral 32) of the primary battery 23 to V.sub.0 /(r.sub.0 +r.sub.m) using a straight line. Note that reference symbol r.sub.m denotes an equivalent resistance obtained while the VIB motor 25 is operated. Reference symbol I.sub.0VIB indicated by reference numeral 35 is the minimum operation current of the VIB motor 25. When a current larger than I.sub.0VIB flows in the VIB motor 25, the VIB motor 25 is operated; when a current smaller than I.sub.0VIB flows in the VIB motor 25, the VIB motor 25 is not operated. More specifically, since a current at an operating point A which is a crossing point between the V.sub.CE -I.sub.C characteristic curve of the Tr 28 and the DC load line is larger than I.sub.0VIB, the VIB motor 25 is operated. Since a current at an operating point B is smaller than I.sub.0VIB, the VIB motor 25 is not operated. The power supply voltage Vcc of the current amplifier 24 is expressed by Vcc=V.sub.0 -r.sub.0 .times.I.sub.VIB. For this reason, when the voltage Vcc decreases because voltage drop occurs due to a current I.sub.VIB flowing in the VIB motor 25, the operating point is shifted to the operating point B, and the VIB motor 25 may not be operated. Therefore, even though the power supply voltage of the pager may be sufficiently high, the VIB motor does not operate when the voltage of the primary battery 23 is insufficient.